Consumers value the ability to see the contents of their packages. Consumers also appreciate the toughness and gloss of containers made from polyester. Because of this combination of attributes, containers made from polyethylene terephthalate (PET) produced by the injection stretch blow molding process (ISBM) are the most common type of transparent container on the market. However, ISBM processes are generally limited to production of uniformly shaped containers and cannot produce bottles that include various types of handles, such as, for example, a through-handle. Oftentimes, handles are desirable in larger-volume bottles and containers, where gripping a round or square container is cumbersome for the end user. Larger-volume bottles containing a through-handle are most efficiently produced by extrusion blow molding (EBM) processes.
A typical extrusion blow molding process begins with the step of melting a polymeric material in an extruder to produce a molten resin that can then be extruded through a die to thereby form a tube of molten polymer (i.e., a parison). Two mold halves having the shape of the desired container are clamped around the parison and, thereafter, air or other pressurized fluid is blown into the mold to expand the extrudate to fill the interior of the mold cavity. The molded article is then cooled and ejected from the mold. Finally, excess polymeric material remaining around the edges and base of the container, typically referred to as “flash” can be removed via various mechanical means before using, shipping, or storing the final container.
Unfortunately, extrusion blow molded containers made from certain polyester resins tend to be rigid, brittle, and challenging to trim. Extrusion blow molded containers made from these types of polyesters tend to have a poor drop impact performance, due, at least in part, to the high degree of rigidity of the final container. In contrast, extrusion blow molded containers produced using polyolefins, such as high-density polyethylene (HDPE), exhibit less rigidity and tend to exhibit a better drop impact strength.
Many factors can influence the drop impact integrity of an extrusion blow molded container, such as, for example, the melt temperature of the parison, the mold temperature of the container and flash pocket, the mold closing speed (also known as the mold cushion level), the spacing between the mold halves (also known as the stand-off or pinch-off distance), the time between ejection of the container and its deflashing, the cooling time in the mold (also known as the mold cycle time), and/or the specific structural configuration of each of the mold halves. In some cases, choosing to alter one or more of the above-listed parameters in order to enhance drop impact strength actually results in a decline or degradation of one or more properties of the final container. Thus, optimizing the drop impact strength of an extrusion blow molded container is a very complex process that is difficult to achieve. For example, although an increase in the cooling time or time-to-deflash of a certain container may increase the drop impact strength, the longer process time required to produce the container would decrease the economic viability of the process, especially on a commercial scale. Similarly, raising the melt temperature of the polymer being used to create the container may increase its strength, but such a change often results in poor parison stability and/or increased polymer degradation during processing.
In addition, while increasing the mold half spacing during processing may increase the drop impact strength, such a modification will also drastically increase the effort required to remove the “flash,” or excess polymer, from the periphery of the finished container (e.g., the “torque to deflash”). Conversely, reducing the spacing between mold halves may cause the resulting flash to trim more easily, but such a change will cause the drop impact strength of the resulting container to suffer.
Thus, a need exists for a process and a system for producing durable extrusion blow molded containers having high drop impact strength yet are easy to deflash (or trim) Such containers should be able to be produced cost efficiently on a commercial scale.